This invention generally relates to method and apparatus for reducing health risks associated with radon, and more particularly relates to removing both attached and unattached radon decay products from air.
As is well known, radon is a naturally occurring radioactive gas that is one stage of a natural radioactive process, known as decay, where larger, more complex elements such as uranium and radium break down into smaller elements and energetic particles. More specifically, the radioactive decay of radium 226 produces the inert radioactive gas radon which is free to diffuse through porous soils and construction materials and eventually enter the environment within a building.
As is also well known, high levels of radon gas can lead to a health hazard, and this is especially true in modern energy efficient homes where levels can build up because there is little exchange of indoor and outdoor air. Radon gas itself is relatively harmless. However, when radon gas decays, a series of radon decay products commonly referred to as radon daughters or daughter particles are created, and alpha and beta particles are emitted. The radon decay products can readily attach themselves to airborne particles such as dust and, in such state, are commonly referred to as attached radon decay products or daughter particles. The effective size of such attached particles ranges from 0.05 to a few micrometers in diameter. Depending on the concentration of dust or similar airborne particles in the air, up to 20% of the radon daughter particles or decay products will not attach to dust particles and remain in a free ion state typically having a diameter of less than 3.0 nanometers. Whether the radon decay products are attached or unattached, they only pose a serious health risk if they are inhaled into the lungs where they can emit damaging particles into nearby cell tissue. Therefore, it is desirable to remove both attached and unattached radon decay products from the air before they are inhaled. Various prior art methods or approaches have been used in an attempt to remove airborne radon decay products.
One prior art approach for removing airborne radon decay products is to pass room air through an air filter such as a conventional electrostatic filter or a filtration device that collects particles on a filter media such as fiber glass matting. This approach, however, has serious drawbacks. The attached radon decay products may be effectively removed because they are attached to relatively large airborne particles such as dust which may, as described above, have an effective diameter of 0.05 micrometers or larger. However, such air filters are generally not effective for filtering out the much smaller unattached radon decay products having a diameter of 3.0 nanometers or less. Further, because the filtering process removes dust from the air, there are fewer airborne particles such as dust to which the unattached radon decay products can attach; therefore, the concentration of unattached particles may actually increase. It is further believed that the unattached radon decay products are more harmful or potentially more dangerous than attached radon decay products because the smaller unattached particles have a higher probability of being deposited deep into the lungs. In contrast, the larger attached particles have a higher probability of being deposited in the upper lung where they can be cleared away by the ciliated mucus lining. Therefore, the net effect of removing attached particles and increasing the concentration of unattached particles may actually increase the radon health hazard.
Another prior art approach described in U.S. Pat. No. 4,596,585 issued Jan. 24, 1986 to Moeller et al. uses a tandem approach combining the two air treatment methods of (a) turbulent convection using a ceiling fan and (b) unipolar space charging employing an ion generator. Specifically, the tandem air treatment method is implemented by directly coupling an air ionizer with a ceiling fan. The highly diffusive nature of unattached particles in the size range of 0.001 to 0.01 micrometers favors their removal from the air by deposition or plating-out as a result of molecular diffusion. The fan attempts to enhance this process by creating air turbulence that has been reported to reduce the boundary layer thickness at the surface to air interface throughout a room and reducing the distance that unattached decay products must travel by molecular diffusion before depositing onto room surfaces. The net result is a higher flux of unattached decay products plating onto the walls of the room and a corresponding reduction in the airborne concentrations of such unattached decay products. The air ionizer portion of the tandem was reported to operate by providing mutual repulsion of the air ions in the vicinity of a strong unipolar point source thereby creating a spatial distribution of airborne charge and an electric field gradient directed radially from the source. By the process of diffusion charging, decay product atoms as well as airborne particles to which some decay products are attached, become charged to the polarity of the ion generator. The force exerted by the electric field on these charged particles causes their migration towards the boundaries of the air space thereby resulting in deposition or plating-out on wall or ceiling surfaces.
The method and radon removing apparatus described in U.S. Pat. No. 4,596,585 have drawbacks. First, with such arrangement, the rate of plate out is very dependent upon the physical parameters of the room such as its size, layout, texture of surfaces, etc. Therefore, the removal rate of radon decay products is also very dependent on such room parameters, and it is undesirable to have to match the radon removing apparatus to a particular room. Further, the walls and ceiling are necessarily spaced from the radon removing apparatus by a relatively large distance. Therefore, a relatively high voltage charge has to be used, and even then the removal of radon decay products may be relatively ineffective due to the large distance. Another problem is that airborne particles such as smoke and dust particles are ionized and removed from the air by plating-out on room surfaces. This leaves fewer airborne particles for the more dangerous unattached radon decay products to attach. Thus, the concentration of the more harmful airborne unattached particles may not be effectively reduced.